Quote Originally Posted by Charlie View Post
The voltage across the transistor when it's on will be 9V - 4.3V = 4.7V Multiply this by the current flowing to get the power dissipated by the device, and if it is too high, then choose a resistor, based on the maximum current, and put it between the collector and 9V to move some power out of the transistor and into the resistor. (I doubt you will need to do this).
So in essence, I'm moving the 9V->5V 'conversion heat' dissapation from the regulator, directly to the tranny controlling the LEDs?

Perhaps I'd be better just upping the spec of the regulator?

I guess what I was shooting for was the most battery friendly way of driving six LEDs from 7.5V thru 9V, and in my simplistic world I thought that avoiding regulating the 9V down to 5V (direct heat loss), and using PWM to switch the tranny on/off would be the way to go!